Preview size of the PDF files in the article would vary for browsers. However you can configure the desired width of the PDF file.
Solution
To enlarge the pdf perview:
- Go to Settings -> Knowledge base site -> Custom CSS & Javascript, and Custom CSS tab is the default selection
- Paste the below CSS snippet in the window
.content_block .content_container .content_block_text p * {width: 100%;}
- Click Save